Pressure responsive instrument



Nov'. 3, 1942.A J. w ROBINS 2,300,810.

PRES SURE RESPONS IVE INSTRUMENT Filed June 26, 1941 3' Sheets-Sheetlf`-`/ l A I Nov. 3, 1942.

.1. w. RoBlNS f PRESSURE RESPONSIVE INSTRUMENT Filed June 26, 1941 3Sheets-Sheet 2 Nv. 3, 1942. J. w. RoBlNs V 2,300,810

PRESSURE`RESPONSIVE INSTRUMENT f vFiled June 26, 1941 5 Sheets-Sheet 3Patented Nov. 3, 1942 UNITE-D STAT-Es PATENT oFFlcE 2,300,810 v PRESSURERESPONSIVE INSTRUMENT .101m W. mbins, Fairfield, conn., assignr toManning, Maxwell & Moore, Incorporated, New York, N. Y., a corporationof New Jersey Application June 26, 1941, Serial No. 399,911

8 Claims.

This invention pertains to pressure-responsive -instruments, forinstance pressure gauges, dial thermometers, etc., and Yrelates moreparticularly to an instrument so designed as tobe substantiallyindependent in its action of barometric changes, the embodiment herechosen for illustration being an'v absolute pressure gauge for measuringairplane. engine manifold pressures. However, the invention is broadlyapplicable to 'other uses and is readily capable of embodiment 10 mustpossess characteristics not necessarily required in an instrument of thesame general kind employed for industrial purposes. Thus limitation ofspace in an airplane necessitates the use of an instrument of minimumdimensions; the

wide variation in the angle of flight of an airplane makes it essentialthat any instrument employed must be substantially unaffected by itsposition in space; the shocksof landing and thee. vibrations ofoperation of the plane must not substantially aect the operation of/theinstru- Vment and, since the safety of the plane and of its passengersmay be lentirely dependent upon the indications of the particularinstrument, the latter must be extremely accurate and dependable. underall conditions of use, including wide variations in temperature.

The present invention has for its principal object the provision of aninstrument of the class described which, in respect to dimensions,durabilityand accuracy in any position in space, is

acceptable for use in an airplane, and which during such use issubstantially unaifected by the shocks and Vibrations to which it may besubjected. A further object is to provide a manifold gauge for use withan airplane engine which is not responsive to changes in barometricpressure, and-which is of great durability.

A further object is to provide a manifold gauge for use with airplaneengines in which the movable parts and dial are exposed to the airrather than to gasoline vapor, and which can be depended upon tomaintain a substantially uniform degree of .sensitivity to pressurevariations throughout a long period of use.

A further obect is to provide a pressure ga'uge 50 of that type whereinopposed pressure-responsive elements are employed vfor actuating asingle pointer or index but wherein said elements are so designed andarranged that the instrument in which the inaccuracies common to priorgauges employing opposed pressure-responsive elements are substantiallyeliminated.

A further object is to provide an instrument of the class describedhaving a springas one of its essential elements but so designed that notonly is the spring modulus substantially unaffected by temperaturechanges but the accuracy of reading of the instrument is likewisesubstantially unaffected by diiferential expansion of the variousconstituent parts of the instrument.

Other and further objects and advantages of ythe invention may bepointed out in the following more detailed descriptiog` and by referenceto the accompanying drawings, wherein Fig.Y 1 is a fragmentary frontelevation of a pressure gauge designed 5to indicate absolute pressuresand emboding the present invention,l

portions of the case and dial being broken away to-show the -movementmechanism and the pressure-responsive elements;

Fig. 2 is a plan'view of the movement mecha- `nism andpressure-responsive.elements removed from the case, certain parts beingin horizontal section;

Fig. 3 is a front elevation of the parts shown in Fig. 2, omitting the.dial, and with certain parts in vertical section;

Fig. 4 is a vertical section substantially on the line 4-4 of Fig. 3; Y

Fig. 5 is a fragmentary front elevation illustrating a modified andpreferred form of motiontransmitting means designed to compensate fordifferential expansion of the parts; and

Fig. 6 is an elevation of the parts shown in Fig. 5, viewed from theleft-.handy side of the latter, with certain parts in vertical section.

A majority of modern aircraft enginesare now provided with superchargerapparatus in order to maintain the desired power output atliighaltitudes, the general practice being to supercharge the fuel-airmixture. Since the Weight of the fuel which .can be ignited at eachstroke depends on the weight of air available, the power output at highaltitudes can only be maintained by increasing the absolute pressure ofthe air delivered to the cylinders. Assuming that other factors remainconstant, the weight of air per unit volume is directly` proportional toits absolute pressure. Hence, for lmeasuring the supercharger (i. e.,manifold) pressure, it is necessary to employ an absolute pressure gaugesince the ordinary pressure gauge indicates gauge presneed not be ofundue size for airplane use and 5,5 sure, that is to say, therdifference between the measured pressure and the external barometricpressure.

Certain prior types of manifold pressure gauges have employed anevacuated aneroid diaphragm capsule mounted within a sealed case. Themanifold pressure is admitted to the inside of this sealed case and theresultant deection of the capsule is transmitted to a suitable indicatorby conventional gauge movement mechanism. While this prior constructionis very simple, it has the disadvantage that the movement mechanism anddial are exposed to gasoline vapor and further that it is difficult tomaintain the accuracy of operation of the parts over long periods ofuse. The mechanism of theprese'nt invention, as now to be described,avoids these previous disadvantages and provides an instru- -ment whichis accurate and dependable and which is not unduly complicated, while atthe same time possessing further advantages hereinafter referred to notcommon to instruments of this type as previously commercialized.

Aircraft manifold pressure gauges are subjected during use to widelyvarying temperatures, and it is now the standard practice to specifythat such instruments meet certain temperature tests. In general thespecications for temperature tests permit onlya very small variation incalibration, when subjected to temperatures varying between -35 C. and+70 C., from the calibration of the instrument at room temperature.

'I'he calibration errors which result from changes in temperature are ingeneral of two types-first, those resultant from changes in the springmodulus (when a spring is employed as an element of the mechanism) andsecond, changes due to differential thermal expansion of materialsemployed in the instrument. In accordance with the present invention,errors in reading of the instrument by reason of temperature variationhave been substantially eliminated.

Referring to the drawings, the numeral I designates a portion of thecase of the instrument, such case being of any desired form andconstruction, althoughit is contemplated that for most purposes va casesimilar to that of the conventional pressure gauge will be employed.Such a case usually comprises a substantially cylindrical side wall andrear wall of pressed metalor the like and is provided with a removableront cap or cover having a transparent panel which protects themechanism within the case and at the same time exposes the dial andindicator to View.

In accordance with the present invention, and as here illustrated, theimproved instrument has a frame comprising the rear plate 2 v(Figs. 3and 4) which may be mounted within the gauge case in any desired manner,forexample by the employment of screws or bolts bywhich it is attachedto the inner'surface of the rear wall of the case. This plate isprovided withl an opening which receives the inner or forward end of aplug 3 (Figs. 2 and 4) having an` internally screwthreaded socket 4designed to receive the end of a conduit which leads from the manifoldof the engine. This plug may be secured to the -plate 2 in any desiredway, =for example by screw threads, welding or the like. While as hereshown this plug with its socket 4 is arranged at the rear of theapparatus, it is to be understood that the latter fitting within asocket in the plug 3 ing instead of through the rear wall as hereprovided for. The inner end of the socket 4 communicates with a chamber5 (Fig. 2) within the plug 3 and from this chamber leads a lateralpassage 6, the entrance to which may be con-J The passage 6 trolled byan adjustable plug 1. is aligned with a canal 8 in a conduit 9, the endof wherein it may be permanently secured by welding or other appropriatemeans.

The supporting frame of the instrument Ialso comprises a pair ofparallel substantially horizontal bars I0 and II (Fig. 4) spaced,apartvertically and which may be secured to the plate 2 inl any desiredmanner, for example by screws,

bolts or welding. To the. opposite ends of these Y bars I0 and II aresecured forwardly extending, parallel plates I3 and I4 (Fig. 2). Spacedforwardly of the bars I0 and II and in the. horizontal planes of theserespective bars are two other bars4 I5 and I6 (Fig. 4) whose ends aresecured to the plates I3 and I4 and which, with the bars I0 and II,define a substantially rectan-l gular, transversely elongate chamber forthe reception of the pressure-responsive elements hereinafter described.'I'he plates I3 and I4 extend forwardly of the bars I5 and I6 and arepreferably provided with flanges I1 and I8 (Fig. 2) respectively attheir forward ends, such flanges providing a suitable support to whichthe dial I9 is secured. This dial is suitably graduated for cooperationwith the movable index or pointer 20. Within the aforesaid space definedby the barsv in position against the plate I3 by means of a cap 25,having openings for `thereception of clamping screws 25* which engagethreaded bores in the boss 24. The head 26 of the bellows ordiaphragmt22 is likewise provided with a central'boss `21 which fits inan opening in the plate I4, the head 29 being clamped to the plate I4 bythe cap 28.

'I'he boss 24 of the head 23 is provided with a passage 29 leading tothe interior of the bellows 2| and the end of the conduit 9 is securedleak- K tight in a socket in' the boss 24 with its interior' canal 8communicating with the passage 29.

'Ihe boss 21 of the head 26 is likewise provided with'a passage 30 whichcommunicates with th'e interior of the bellows 2'2, and a flexibleconduit 3l has one end fixed in a socket in the boss 21 with itsinterior communicating with the passage 30 and has its opposite endsealed as indicated at 32, after completion of the instrument.

During manufacture ofthe. instrument and assuming that bellows 22 isto'v be evacuated, the

' conduit `3I is connected to the air pump and when the desi'red vacuumhas been'obtained the conduit 3| is sealed, for example by welding at32, thus preventing the entrance of atmospheric pressure into thebellows 22. `On the other hand, if the bellows 22 is to be supplied withpressure "-fiuid from a source of varying pressure, the conit may belocated in any other convenient place,

for example'so that the plug 3 may be extended through an opening in theside wall of the caS- duit 3I will not`be thus sealedv but will lead tosuch'source of varying pressure.

bellows 2| has the movable head 33 and the bellows 22 has the movablehead 34, the areas of the heads 33 and 34 which are exposed to pressurefluid being substantially equal, and the bellows are so arranged thatwere the internal pressure in either bellows to increase, its head wouldtend to move toward the head of the other bellows. The head 33 of thebellows 2| is provided with a central -boss 35 (Fig. 3) having an axial,screw-threaded socket which receives a screw-threaded end 31 of a lengthof spring wire which is coiled to provide the helical tension spring 38.This spring is housed within the bellows 2| and is coaxial with thelatter. The other end 39 of the wire which forms the spring is screwthreaded and received in a screwthreaded axial socket in the boss 24 ofthe xed headv 23. As thus arranged the force exerted by this tensionspring 38 is directed along the axis of the bellows 2| and issubstantially devoid of any component tending to deflect the bellows.The movable heads 33 and34 of the two bellows devices are rigidlyconnected by rthe -sleeve member 48 which may be secured to the bellowsheads in any desired manner, for example by screw threading,`welding orthe like. This sleeve is provided with an elongate longitudinal slot 4|(Figs. 3 and 4) at its upper part for` a purpose hereinafter described,andis preferably .provided with other slots symmetrical with the slot 4|in order to decrease its weight and te'y avoid unbalancing the parts. b

A bridge member 42 (Fig. 2)` is secured to the bars I0 and I5. Thisbridge member is provided with a. pair of spaced lateral brackets 43 and44 (Figs. 2 and 4) (preferably split) 4having internally screw-threadedcoaxial bores -for the reception of the pintle screws 45 and 46 (Fig.4),

respectively. These pintlescrews have pointed or substantially conicalinner ends which are received in conical sockets inthe opposite ends ofa rock shaft 41. By turning the screws 45 and 46 the rock shaft 41 ,maybe accurately adjusted lengthwise. By properly setting the lscrews 45and 46 substantially all longitudinal and transverse motion may beeliminated, thus restricting the motion of the shaft to apure rockingmovement.

The rock shaft-41 is provided at one end with a rigid/'crank arm 48(Fig. 4) which extends down through the slot 4| in the upper part of thesleeve 48 and which has its lower end disposed transversely of thealigned axes of the bellowsdevices 2| and 22.

A shortllink of exible metallic ribbon 49 (Fig. 3) has one end iixedlylsecured to the lower part of the arm 48 and its other end xedlyattached,

by means of a screw 50, to the boss 35 of the movable head 33 of thebellows 2|. This flexible link 48 is preferably arranged in thehorizontal plane of the axis of the bellows 2|. Since this link isrigidly secured at one end to the arm 48 and atthe other to the boss35,` all lost motion between vthe. parts 35 and 48 is eliminated,although the exibility of the link permits the arm 48 to swing in an arcabout the axis of thev shaft 41 without substantially defiecting'thehead 33 of the bellows from its normal vertical plane.

Were there any lost motion at this point such as to the frictiondeveloped at the bearings.

jointless flexible link here employed completelyxwhen subjected tovibration the inertia forces exerted are even greater. Thus to provide abiasing load which would be dependable under all conditions, the Aspringwould necessarily be so strong that the device would be inoperative duevThe eliminates losty motion, and thus makes it unnecessary to employ aloading spring of any kind.

The arm 48 is extended across theaxis of the shaft 41 to form thesecondarm 48EL (Figs. 3 and 4) and at the opposite end of the shaft 41there is provided a parallel crank arm 48b rigidly secured to the shaftand together with the arm 48*l rigidly supporting a counterbalanceweight 5|l disposed eccentrically with respect to the axis of the shaft41. y

The crank arm 48h is provided with a lateral integral offset 52 (Figs. 2and 3) and to this oiset is .pivotally secured a lever 53 (Fig. 3)having an arm 54 which is provided with an arcuate slot which receives aset screw 55 engaging a thread opening in the arm 48h, thus permittingthe lever 53 to be angularly adjusted with respect to the arm 48a.

The lever 53 also has an arm 56 which is connected by means of a link 51to the tailpiece 58. (Fig. nl) of the sector lever 58 of a conventionalgauge movement mechanism which turns the index staff 60 upon which theindex or pointer 20 is mounted. The adjustability of the lever 56stantially as follows: Since the bellows 22 is evacuated the spring 38will always be under tension except in-such rare case as`when thebellows 2| might be under absolute zero pressure. Any increase above`zero pressure in the bellows 2| causes the bellows heads 33 and 34,together with f the spacer sleeve 40, to move toward the right,

and since. the two bellows devices have equal effective areas, thebarometric pressure acting upon the outside surface of the bellowsdevices when the pointer 20 is at the midportion of the would resultfrom the use of joints in the connections between the parts 35 and 48,it would be necessary to bias the-parts always'in one direction to takeup such lost motion, as for example by the use of a loading spring.However, in certain positions of the instrument, the force imposed bythe counterweight is considerable and graduated scale. Thus for exampleif the scale be graduated to read from ten to fifty absolute, asindicated in Fig. 1, then when the pointer is at the graduation thirty,the lever arm 48 will be substantially vertical. Thus the movement ofthis arm from this midportion will make equal angles as the pointermoves either up or down on the scale toward one limit or the other andthus any error due to angularity of the movement of the arm 48 isminimized. Since the flexible link eliminates lost motion between theboss 35 and the arm 48, the possibility of inaccuracy at this point suchas might be caused by the use of pivots springs are employed inconnection with' such bellows. The tilting of a bellows of this type notonly decreases accuracy of operation but is also By thev very injuriousto the bellows themselves. employment of the tension spring so arrangedas to exert its force in a direction which is accurately axial of thebellows, substantially all tilting of the bellows is eliminated and anysuch very slight tilting as may occur is averaged. Moreover, this way ofapplying the spring force insures the maximum force for overcoming thefriction in the rock shaft and other parts and thus decreases suchtendency to tilt the bellows as may result from an overloading of theparts by reason of excessive friction.

The mass of the counterweight 5I is substantially eq'ualto the effectivemass of the bellows, bellows heads, vspacer sleeve, spring and othermoving parts, and this weight 5| is so placed as substantially tobalance all inertia forces which may act along the longitudinal axis ofthe bellows by reason of aircraft vibration. The instrument is thusdynamically balanced. Moreover, by the provision of this weight, theapparatus is also substantially statically balanced thus eliminatingpositional error of the instrument.

In accordance with the present invention, the spring 38 is preferablymade of a material having a substantially zero temperature coeicient ofSpringshaving this characteristic are commercially available and areunderstood to consist of an alloy containing approximately 36% nickel,8% or more chromium, and 4% of other elements, including manganese,silicon, molybdenum, copper and vanadium, the remainder of thealloy'being iron. The employment of such -a spring eliminates most ofthe temperature errors arising from variation in spring modulus, and

although there still remains a slight error due t0 change in the elasticmodulus of the bellows, this error is so slight as to be substantiallynegligible.

However, the employment of a spring having a zero temperaturecoeillcient introduces another diiliculty from the fact that the lengthof the spring is substantially constant as respects temperature changes,whereas other parts, in particular the frame of the instrument, which isstantially identical with the correspondingly numbered parts illustratedin Figs. 1 to 4inclusive, except that thepart 48h, 52 is made of amaterial which has a substantially zero thermal coeillcient ofexpansion. Such a material, commonly known under the trade name Invar,is a commercial product readily available.

A lever 53x is pivotally secured at its lower end to the outer end ofthe member 52, the lever 53X being shown as inclining upwardly andhaving a substantially vertical upper end portion 54x, the lever havingan elongate slot which receives a set screw 55x, thus permitting thelever 53x to be swung about its pivotal connection to the part 52 and tobe xed in adjusted position,

such adjustment being desirable to facilitate calibration of theinstrument.

To the upper end of the lever arm 54x is rigidly secured a member 56xhaving a downwardly sloping elongate arm 56Y, the parts lilix and 56being of brass or other material having a substantial coeiilcient ofthermal expansion, while the lever 53x, like the part 52, is of amaterial having a substantially zero thermal coilicient of expansion.

With this arrangement, in which the part 56X,

. 56y is supported above the axis of 'the shaft 41 ata distance which issubstantially invariable as respects temperature variations, the freeend of the arm 5W, to which the link 51 is secured, has a slight 'butdefinite vertical movement in response to temperature variations. Theparts are so dimensioned and designed that this slight vertical movement'of the end of the arm 561 accurately compensates'for any such slightrocking of the rock shaft 41 as may result from the differential thermalexpansion of the frame and the spring 38. Since the index receives itsmotion through appropriate connections from thefree end of arm 56?, suchcompensation is suicient substantially to prevent any movement of theindex as a result of temperature variations to vwhich the instrument maybe subjected during use.

Such terms as horizontal, vertical, front and rear have been employedfor convenience in description, but it is to be` understood that theseterms are not to be regarded as limiting, since the instrument may beinstalled in any desired position and since in use on an airplane `it isobvious that it may be tilted to various angles and will not alwaysoccupy the same position with respect to the surface of the earth. Whileherein the pressure-responsive elements have been specically illustratedas flexible metallic bellows, it is to be understood that otherequivalent devices, for example Bourdon tubes or ordinary diaphragms maybe arranged in a generally` similar way to accomplish similar results.It is also to be understood, as above suggested, that this instrumentor' one having the same general characteristics may be employed forindicating dierential pressures. in which case both diaphragms orbellows devices will be subjected to varying internal pressures. ,In-

this, as in the arrangement more specifically described above, themovement of the lever 48 is in response to the algebraic sum of theoppositely directed pressure forces acting upon the inner sides of heads23 and 26.

It is to be understood that the invention is not necessarily limited tothe precise arrangement of parts Aherein disclosed but that otherarrangements such as above suggested or otherwise and which fall withinthe scope of the appended claims are to be regarded as within thepurview of the invention.

I claim:

.1. A pressure-responsive instrument ofA the class described comprisinga movable index, pressure motor means including a hollow metallicbellows having a movable head and connections for transmitting motionfrom the head to the index, said connections including a rock shaft,motion-transmitting means actuated by the head and operative to rock theshaft, a tension spring 1 rial having a substantially zero temperaturecoemcient of modulus, the connections 'for transmitting motion from themovable head to the index including a composite crank including a partwhich is normally rigidly secured to and which extends upwardly from therock shaft, and an elongate arm rigidly secured to the free end of saidupwardly extending part and which inclines downwardly toward its freeend, said upwardly extending part being of a material which has asubstantially zero coefcient of expansion and the elongate part having asubstantial coeiiicient of expansion, the parts being so designed andarranged that the position of the free end of said elongate arm in spaceis substantially unchanged by diierential expansion of the spring andthe other parts of the instrument due to temperature changes.

2. An instrument of the class described comprising a, movable index, apressure motor for actuating the index, and connections between themotor and index including a rock shaft which is rocked by the motor, acrank arm rigidly iixed tothe rock shaft, and an elongate arm-which isnormally flxedto but which is ad- Justable with reference to said crankarm, said crank arm being of a material having a substantially zerocoemcient of expansion, and the adjustable arm being of a materialhaving a substantial coefiicient of expansion, the parts being sodesigned and arranged that the position in` coaxial# with and unitingsaid movable heads, said sleeve having an elongate slot in its wall, theframe having aligned internally screw-threaded bores invwhich adjustablepintles aremounted, a rock shaft having sockets in its oppositeendswhich receive the respective pintles, the axis of the rock shaft beingin a plane which is perpendicular to the axes of the bellows devices andbeing spaced from the latter, a crank arm iixed to the shaft, the freeend portion of said crank arm entering the sleeve through the slot inthe latter, flexible jointless connecting means uniting the movable headof one ofsaid bellows devices to the crank arm, a tension spring housedwithin that bellows device to which fluid at varying pressure isadmitted, said spring being arranged to oppose movement of said head inre sponse to increasing internal pressure within said latter bellowsdevice, a motion-transmitting lever having one arm thereof adjustablyconnected to said rock shaft, an index, and means forspace of the. freeend of :said elongate arm is not substantially changed by differentialexpansion of the instrument parts in response to temperature changes.

3. In an instrumentv of the class described,

two axially aligned,.fiexible, metallic bellowseach having a movablehead, the heads being spaced apart and of substantially equal eiecvtivearea and being so arranged that they would move in opposite directionsin response to increase in internal pressure within the respectivebellows, one of said bellows being evacuated and permanently sealed,means operative to admit pressure uid to the other bellows, a tensionspring within the latter bellows arranged to op- 'pose movement of itshead in response to increasing internal pressure, a rigid sleeveconnecting said movable heads, said sleeve having an elongate slot inits wall, a rock shaft having its axis in a plane perpendicular to theaxes of the bellows, the axis of the rock shaft being spaced from thebellows axes and outside of the sleeve, a crank arm xed to said rockshaft, the free end portion of said crank arm entering the connectingsleeve through the slot in the latter, said crank arm beingsubstantially perpendicular to the bellows axes when the index is at themid-point of its path of movement, an index, a stifiiy ilexible linksubstantially aligned with the bellows axes, one end of the link beingrigidly secured to one of said movable heads and its oppoy, site endbeing rigidly secured to the crank arm,

w apartand opposed to each other, a rigid sleeve transmitting movementof said latter lever to the index.

5. In an instrument of the class described wherein an index is movableover a graduated dial by motor means comprising two axially alignedflexible metallic bellows having their axes aligned and normallysubstantially horizontal, each bellows having 4a movable head, thebellows being so arranged that the movable heads 'would move in oppositedirections Ain response to increase in internal pressure in therespective bellows, one of said bellows being evaculate and permanentlysealed, and means operative to admit iiuid at varying pressures to theother bellows, rigid means connecting said movable heads, a tensionspring disposed within l y sponse to increase in internal pressure inthe bellows, a lever supported to rock about an axis substantially'perpendicular to the vertical plane of the bellows axes and spaced fromthe horizontal plane of the bellows axes, said lever being substantiallyvertical when the index is at substantially the mid-point of its travel,means connecting the free end of the lever to one of the movable bellowsheads, means for transmitting motion from the lever to the. index, thelever `comprising a rigid arm. -extending upwardly above its pivotalaxis and a counter-weight mounted upon said latter arm .saidcounterweight being of such mass as substantially to balance all of theother moving parts of the instrument both statically and dynamically.

6. In an instrument of the class described, a

y movable index, a pressure motor having a movable part,and a springwhich constantly urges said part in one direction, said spring 'being soconstructed and arranged as to have a substantially zero temperaturecoeiiicient of modulus, and means for transmitting motion from saidmovable motor part to the index, said motiontransmitting meanscomprising a lever, a exible link rigidly secured at one end to saidmovable motor part and .at its other to the free end of said lever, saidlever having a rigid arm extending to the opposite side of its pivotalaxis, said arm having a lateral offset portion, a. motiontransmitting`lever pivotally -supported on said oiset portion, means for retainingsaid motiontransmitting lever in adjusted angular relation to saidoffset portion, said offset portion being of a material having asubstantially zero coefcient of temperature expansion, and themotion-transmitting lever being of a metal having a substantialcoeflicient of thermal expansion, and means connecting themotion-transmitting lever to the index.

-7. A pressure-responsive instrument of the class described comprising amovable index, a pressure motor including a flexible metallic bellowshaving a movable head and connections for transmitting motion from saidhead to the index, said connections comprising a rock-shaft,motion-transmitting means actuated by the head and operative to exertforce along the axis of the bellows thereby to rock the shaft, a tensionspring arranged to exert force along the axis of the bellows so as tooppose motion of its mov- -able head in response to increase in internalpressure within the bellows, the spring being so constructed andarranged as to have a substantially zero temperature coefllcientof'modulus, the connections for. transmitting motion from the bellowshead to the index including a lever xed to the rock-shaft, said leverhaving oppositely extending arms -to one of which the motion of thebellows head is transmitted thereby to rock the rock shaft, the other ofsaid lever arms having a lateral offset consisting of a material havinga substantially zero coeflicient of thermal expansion, amotion-transmitting lever mounted on said offset, saidmotion-transmitting lever being of a material having a substantialcoeicient of thermal expansion, means for adjustably xing saidmotion-transmitting lever relatively to said` offset, and means fortransmitting motion from said motion-transmitting lever to the index.

8. In an instrument of the class described wherein an index is movableover a graduated dial by motor means comprising two axially alignedflexible metallic bellows having their axes` aligned and normallysubstantially horizontal, each bellows having a movable head, thebellows being so arranged that the movable heads would move in oppositedirections in reone of said bellows to which pressure fluid atvaryingpressures is admitted, said spring being arranged to act axially of saidlatter bellows to oppose movement of its movable head in response toincrease in internal pressure in the bellows, a rock shaft .whose axisis substantially perpendicular to the vertical plane of the bellows axisand spaced from the horizontal plane of the bellows axis, conicallyended pintles engaging conical sockets in the opposite ends,respectively, 'of the rock shaft and constituting rigid supports for theshaft permitting the latter to turn freely but minimizing all othermotion of the shaft, a rigid arm ilxed to the rock shaft, the free endof the arm being adjacent to the axis of the bellows, a lflexible linkrigidly secured at one end to said arm and rigidly connected at itsopposite end to one of the bellows heads, and

a counterweight spaced from and rigidly con-l nected to the rock shaft,said counterweight being of such mass and so spaced from -the axis ofthe' rock shaft as substantially -to balance the bellows assembly bothstatically and dynamically.

JOHN W. ROBINS.

